#include<iostream>

using namespace std;
int main()
{
	
	int n,d;//n站点，d公里/升
	
	cin >> n>> d;
	long long juli[n] = {0};
	long long jiage[n] = {0};
	for(int i = 1;i<n;i++)
	{
		int tmp; 
		cin>>tmp;
		juli[i] = juli[i-1]+tmp;
	}
	
	for(int i = 0;i<n;i++)
	{
		cin>>jiage[i];
	}
	
	long long zj = 0;
	int zhan = 0;
	long long zjl = 0;
	int xiazhan = 0;
	while(zhan < n - 1)
	{
		for(xiazhan = zhan + 1;xiazhan < n - 1;xiazhan++)
		{
			if(jiage[xiazhan]<jiage[zhan])
			{	
				break;
			}
		}
		
		long long jl = juli[xiazhan] - zjl;
		long long you = jl / d;
		if(jl % d != 0)
		{
			you++;
		}
		zj += you * jiage[zhan];
		zjl += you * d;
		zhan = xiazhan;
	}
	
	cout<<zj;
	
	return 0;
}